1. Field of the Invention
This invention relates to the preparation of polyorganosiloxane foams. More particularly, this invention relates to two-part room temperature vulcanizable polyorganosiloxane compositions that include a blowing agent and a novel foam stabilizer. When exposed to atmospheric pressure while still in a flowable state, the combined ingredients of the present compositions yield cured, low density polyorganosiloxane foams without requiring moisture.
2. Background Information
Polyorganosiloxane foam have been prepared using a variety of reactants and conditions. One method utilizes the reaction of an organohydrogensiloxane and a hydroxyl-containing polydiorganosiloxane. This reaction generates hydrogen as a by-product. The hydrogen serves as the blowing agent, making it unnecessary to introduce external blowing agents such as air into the composition. Foams of this type are disclosed in U.S. Pat. No. 3,923,705, issued to Smith on Dec. 2, 1975.
The introduction of air or other gaseous blowing agent into a room temperature curable polyorganosiloxane composition to form a foam that is stabilized during curing by the application of vacuum is taught in U.S. Pat. No. 4,368,279, issued to Modic and Boudreau on Jan. 11, 1983. The polyorganosiloxane compositions can be either one- or two-part. One-part compositions are curable in the presence of moisture and contain a hydroxyl endblocked polydiorganosiloxane together with any of the known moisture-reactive curing agents. The two-part compositions are cured by the platinum catalyzed reaction of silicon-bonded hydrogen atoms with ethylenically unsaturated hydrocarbon radicals or by the reaction of a hydroxyl-endblocked polydiorganosiloxane with a hydrocarbyltrialkoxysilane, an alkyl orthosilicate, or partial hydrolysis products of the silane or orthosilicate, in the presence of a suitable curing catalyst such as a tin compound.
The foams that are generated using any of the compositions disclosed in the aforementioned Modic and Boudreau patent will collapse unless kept in a vacuum chamber until the curing reaction has progressed to the extent that the foam becomes self-supporting. This requires a residence time in the vacuum chamber of at least 15 minutes. The method and compositions taught by Modic and Boudreau are useful for preparing slab stock foam. This method is not practical when a foam is prepared at the site where it is to be ultimately installed, which can be within a conduit or between the walls of a building or other structure. For such foams it would be desirable to transport the ingredients of a foamable composition, including a blowing agent, to the installation site where the ingredients are dispensed as a homogeneous mixture that spontaneously foams and cures to yield a low density product without additional processing.
Most desirably, premeasured amounts of reactants and blowing agent are packaged in one or two pressurized containers such as aerosol cans, and the foam is obtained by dispensing the contents of the container(s) at the location where the foam is to be installed.
The packaging of moisture curable polyorganosiloxane compositions in combination with a compressible gas in the inner compartment of a two-compartment aerosol can is taught in U.S. Pat. No. 4,229,548, issued to Sattleger et al. on Oct. 21, 1980. The outer compartment contains a compressed gas such as air to provide the pressure required to discharge the foamable composition from the storage container. The resultant foams are typically of relatively high density (0.58-0.81 g./cc) due to the presence of the filler required to prevent collapse of partially cured foams.
Additives for reducing the density of polyorganosiloxane foams are taught in the prior art. The use of certain resinous polyorganosiloxanes containing triorganosiloxy and SiO.sub.4/2 units is disclosed in U.S. Pat. No. 4,418,157, issued on Modic on Nov. 29, 1985. U.S. Pat. No. 4,026,845, issued to Kim et al. on May 31, 1977, discloses using known fluorine-containing surfactants for this purpose. In their copending U.S. patent application Ser. No. 665,272 filed on Oct. 26, 1984, now U.S. Pat. No. 4,393,641, the present applicants demonstrate that neither a polyorganosiloxane of Modic or a fluorinated alcohol will prevent collapse of foams prepared by dispensing a mixture of a moisture-curable polyorganosiloxane composition and a blowing agent from an aerosol container.